Salinity Changes and Anoxia Resulting from Enhanced Runoff During the Late Permian Global Warming and Mass Extinction Event
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Clim. Past Discuss., https://doi.org/10.5194/cp-2017-136 Manuscript under review for journal Clim. Past Discussion started: 25 October 2017 c Author(s) 2017. CC BY 4.0 License. 1 Salinity changes and anoxia resulting from enhanced runoff 2 during the late Permian global warming and mass extinction 3 event 4 5 Elsbeth E. van Soelen1, Richard .J Twitchett2, Wolfram M. Kürschner3 6 7 1University of Oslo, Departments of Geosciences, P.O box 1047 Blindern 0316 Oslo, Norway 8 2Natural History Museum, Earth Sciences Department, London, SW7 5BD, UK 9 3Department of Geosciences and Centre for Earth Evolution and Dynamics (CEED), University of Oslo, Oslo, 10 Norway 11 12 Correspondence to: Elsbeth E. van Soelen ([email protected]) 13 1 Clim. Past Discuss., https://doi.org/10.5194/cp-2017-136 Manuscript under review for journal Clim. Past Discussion started: 25 October 2017 c Author(s) 2017. CC BY 4.0 License. 14 Abstract. The Late Permian biotic crisis had a major impact on marine and terrestrial environments. Rising CO2 15 levels following Siberian Trap volcanic activity were likely responsible for expanding marine anoxia and 16 elevated water temperatures. This study focusses on one of the stratigraphically most expanded Permian- 17 Triassic records known, from Jameson land, east Greenland. High resolution sampling allows for a detailed 18 reconstruction of the changing environmental conditions during the extinction event and the development of 19 anoxic water conditions. Since very little is known about how salinity was affected during the extinction event, 20 we especially focus on the aquatic palynomorphs and infer changes in salinity from changes in the assemblage 21 and morphology. The extinction event, here defined by a peak in spore/pollen, indicating disturbance and 22 vegetation destruction in the terrestrial environment, postdates a negative excursion in the total organic carbon, 23 but predates the development of anoxia in the basin. Based on the newest estimations for sedimentation rates, 24 the marine and terrestrial ecosystem collapse took between 1.6 to 8 kyrs, a much shorter interval than previously 25 estimated. The palynofacies and palynomorph records show that the environmental changes can be explained by 26 enhanced runoff, increased primary productivity and water column stratification. A lowering in salinity is 27 supported by changes in the acritarch morphology. The length of the processes of the acritarchs becomes shorter 28 during the extinction event and we propose that these changes are evidence for a reduction in salinity in the 29 shallow marine setting of the study site. This inference is supported by changes in acritarch distribution, which 30 suggest a change in palaeoenvironment from open marine conditions before the start of the extinction event to 31 more near-shore conditions during and after the crisis. In a period of sea-level rise, such a reduction in salinity 32 can only be explained by increased runoff. High amounts of both terrestrial and marine organic fragments in the 33 first anoxic layers suggest that high runoff, increased nutrient availability, possibly in combination with soil 34 erosion, are responsible for the development of anoxia in the basin. Enhanced runoff could result from changes 35 in the hydrological cycle during the late Permian extinction event, which is a likely consequence of global 36 warming. In addition, vegetation destruction and soil erosion may also have resulted in enhanced runoff. 37 Salinity stratification could potentially explain the development of anoxia in other shallow marine sites. The 38 input of fresh water and related changes in coastal salinity could also have implications for the interpretation of 39 oxygen isotope records and sea water temperature reconstructions in some sites. 40 2 Clim. Past Discuss., https://doi.org/10.5194/cp-2017-136 Manuscript under review for journal Clim. Past Discussion started: 25 October 2017 c Author(s) 2017. CC BY 4.0 License. 41 1. Introduction 42 The late Permian extinction event was the most severe global crisis of the Phanerozoic in terms of both 43 taxonomic loss and ecological impact (e.g. McGhee et al., 2012). The current consensus is that the extinction 44 was likely due to global warming and associated environmental changes caused by CO2 emissions from Siberian 45 Trap volcanic activity, because of the close timing between the volcanic activity and the extinction event (e.g., 46 Burgess et al., 2017; Burgess and Bowring, 2015). Most studies on the late Permian extinction have inferred that 47 expanding marine anoxia (e.g. Wignall and Hallam, 1992) is a key biotic factor causing marine extinction and 48 ecosystem collapse. There are different theories to explain the spreading of anoxia, which affected both deep 49 and shallow sites (e.g. Bond and Wignall, 2010; Isozaki, 1997; Wignall and Twitchett, 1996). A weakened 50 temperature gradient between equator and pole would have slowed ocean circulation and may have facilitated 51 expansion of the oxygen minimum zone (Hotinski et al., 2001). Increased weathering and detrital input (Algeo 52 and Twitchett, 2010), and soil erosion (Sephton et al., 2005) led to enhanced terrestrial matter input in marine 53 sections, and may also have contributed to eutrophication and in stratified waters led to hypoxia or anoxia 54 (Sephton et al., 2005). Other factors that are thought to play important roles in the extinction are elevated water 55 temperatures (e.g. Sun et al., 2012), and ocean acidification (Clapham and Payne, 2011). One important 56 environmental parameter that has received relatively little attention is salinity, even though low salinity ocean 57 conditions were once considered to be a leading cause of the marine extinction (Fischer, 1964; Stevens, 1977). 58 Furthermore, potential impacts of changes in salinity, which might be expected from enhanced discharge of 59 freshwater into shelf seas (Winguth and Winguth, 2012), have been largely ignored. 60 Microfossils of marine algae are excellent recorders of environmental change in the water column. Of 61 these, the organic-walled cysts of dinoflagellates have proven especially useful in palaeo-environmental studies 62 (e.g. Ellegaard, 2000; Mertens et al., 2009, 2012, Mudie et al., 2001, 2002; Sluijs and Brinkhuis, 2009; Vernal et 63 al., 2000). While dinocysts are absent or sparse in Palaeozoic deposits, acritarchs are commonly recorded (e.g. 64 Tappan and Loeblich, 1973), even during the late Permian when acritarch diversity was declining (Lei et al., 65 2013b). Acritarchs are a group of microfossils of organic composition and unknown affinity (Evitt, 1963). Many 66 acritarchs are, however, found exclusively in marine rocks, and some are thought to be precursors of modern 67 dinoflagellate cysts (e.g. Servais et al., 2004). Several studies in East Greenland have reported fluctuating 68 abundances of acritarchs during the Late Permian and Early Triassic (e.g. Balme, 1979; Piasecki, 1984; 69 Stemmerik et al., 2001), but very few late Permian studies have documented the relative abundance of the 70 different genera of aquatic palynomorphs (for example, Shen et al., 2013). Similar to dinocysts, acritarch 71 morphology has been linked to environmental conditions, and acritarchs with longer processes are generally 72 more abundant in more open marine settings (Lei et al., 2012; Stricanne et al., 2004). Both dinocyst and 73 acritarch studies show that salinity is an important, albeit not the only, factor that influences cyst morphology. It 74 is thought that the longer processes in higher salinities stimulate clustering with other cysts or particles in the 75 water column, and thus enhance sinking to the seafloor (Mertens et al., 2009). 76 The rock record of Jameson Land, East Greenland, has provided key insights into marine environmental 77 changes during the late Permian extinction event. Previous work on this section by Twitchett et al. (2001) 78 showed that collapse of marine and terrestrial ecosystems was synchronous and took between 10 to 60 kyr. 79 Palynological work by Looy et al. (2001) showed that the terrestrial ecosystem collapse is characterized by a 3 Clim. Past Discuss., https://doi.org/10.5194/cp-2017-136 Manuscript under review for journal Clim. Past Discussion started: 25 October 2017 c Author(s) 2017. CC BY 4.0 License. 80 distinct rise in spores, indicative of the loss of woodland and increase in disturbance taxa. The onset of anoxic 81 conditions is not as abrupt at this location as seen in some other sections, but instead Wignall and Twitchett, 82 (2002) found unusual alternating patterns of bioturbated and laminated siltstones in the top of the Schuchert Dal 83 Formation and lowest of the Wordie Creek Formation (Fig. 1). In a recent study, Mettam et al., (2017) showed 84 that rapidly fluctuating redox conditions occurred during the late Permian extinction, while sedimentological 85 observations imply that sea level was rising in this period. Estimations of sedimentation rates indicate that each 86 bioturbated/laminated rock interval, which are between 2-10 cm thick, have been deposited in a period of 50- 87 1000 years (Mettam et al., 2017). To study the environmental conditions associated with the deposition of these 88 sediments we look in detail at a short (9 m) interval covering the late Permian extinction and the Formation 89 boundary between the Schuchert Dal and Wordie Creek Formation. Palaeoenvironment is studied by looking at 90 variations in the palynofacies (organic particles) and the acritarch assemblage. In addition we present a record of 91 morphological changes within the acritarch Micrhystridium. Runoff and erosion, salinity, sea-level fluctuations 92 and temperature rise are discussed as possible reasons for the changing environmental conditions. 93 94 2. Geological setting and stratigraphy 95 Samples were collected at the Fiskegrav location of Stemmerik et al. (2001), an outcrop in East Greenland 96 (N71°32’01.6ˮ, W024°20’03.0ˮ), in a small stream section on the east side of Schuchert Dal (Fig.